In recent years, there have been increasing demands for multiband or multifunctional wireless terminal devices such as mobile phones and the like. Herein, “multiband” means that wireless communication is possible using various frequency bands. Meanwhile, there have also been increasing demands to reduce the size and cost of wireless terminal devices. Therefore, there has been a demand to reduce the number of parts so as to allow many bands to be utilized and many functions to be supported with smaller number of parts.
A wireless terminal device includes a baseband (BB) unit, a power amplifier (PA), a low noise amplifier (LNA), and a duplexer (DUP). The BB unit performs modulation of a transmit signal, and demodulation of a receive signal. The PA amplifies the transmit signal. The LNA amplifies a receive signal. The DUP performs isolation receive signal from transmit and receive signal. And DUP also performs synthesis of the transmit signal and the receive signal. In a wireless terminal device which uses a plurality of bands, the numbers of BB units, PAs, LNAs, and DUPs are the same as the number of bands (frequency bands) used. In such a wireless terminal device which uses a plurality of bands, the number of antennas may be less than the number of bands used. In that case, the wireless terminal device includes a switch between antenna and some DUPs corresponding to each band. The switch connected antenna and the DUP corresponding to the band to be used. In that way, the wireless terminal device performs wireless communication using each band. In one conventional technique regarding this switch, a switch is used as an attenuator by setting a control voltage of the switch to an intermediate value.
Also, in wireless terminal devices, some cases, an isolator (ISO) is arranged between a PA and a DUP so that the PA is not influenced by a fluctuation in antenna impedance. It was caused by change in position of an antenna or the like. Since there have been increased demands to reduce the number of parts in recent years as described above, removal of the ISO has been of interest.
When an ISO is not used in a wireless terminal device, the impedance of an antenna is directly apparent as the load impedance of a PA. Therefore, the PA is influenced by the load impedance of the antenna.
In a clamshell wireless terminal device or the like, a casing can be in two states, i.e., an open state and a closed state. In such a wireless terminal device, the position of an antenna of the casing changes according to each state. Specifically, the antenna is located approximately in the center of the casing when the casing is open, whereas the antenna is located in an end portion of the casing when the casing is closed. When the position of the antenna with respect to the casing changes, the impedance of the antenna also changes. Therefore, a technique for preventing a PA from being influenced by a change in impedance of the antenna is used for a wireless terminal device without an ISO.
Conventionally, a technique has been proposed for when the impedance of an antenna varies, in which a backoff from a saturation region enables an operation region of a PA to be used with increased load stability in order to ensure the device characteristic even if the impedance varies.    Patent Document 1: Japanese Laid-open Patent Publication No. 9-270659
However, a PA with a large amount of backoff is less power-efficient and requires more quiescent current compared to a PA with a small backoff. Therefore, it has been difficult to combine high power efficiency and reducing the number of parts by not using an ISO.